1. Technical Field
The present invention is directed to the art of power supply systems and, more particularly, to improvements relating to multiple phase power line power supplies having current shared power factor correction.
2. Description of the Prior Art
Low power line disturbance power supply is becoming a preferred power supply for all new electronic circuits. A low power line disturbance power supply generates very small harmonic current feedbacks to the power grid.
The common technique that is employed today is called power factor correction (PFC), where a boost regulator (PFC pre-regulator) is used to pre-regulate the output voltage while forcing the input current to be sinusoidal and also in phase with the input line voltage. When the voltage and current are in phase, the power factor (PF) can be approaching unity and therefore, the real power is equal to the apparent power. This technique is called Active Power Factor Correction regulator, and the technique is only applicable to a single phase power line system.
To complete the power supply system, a high frequency DC/DC converter is added to provide a regulated DC output voltage to the load. See FIG. 1. A low power line disturbance power supply consists a front end power factor correction regulator followed by a switching voltage regulator.
As shown in FIG. 1, it is known in the prior art to employ a single phase system for an AC supply wherein an input rectifier filter 10 receives one phase of an AC signal and provides a rectified signal to a power factor correction pre-regulator circuit 12. This circuit pre-regulates the output voltage while forcing the input current to be sinusoidal and also to be in phase with the input line voltage. The output from regulator 12 is a regulated voltage which is applied to a high frequency DC-to-DC converter 14 which then supplies a regulated DC output voltage to a load 16.
As power requirement increases, a single phase power supply system is limited in its power output. A three phase power line system is necessary to handle higher output power.
In order to obtain a three phase power line disturbance power supply, three separate identical active power factor correction power supply systems are used and followed by a current sharing loop to balance current between these three power supplies. See FIG. 2. These three power supply systems are ORed together to form a single output sourcing current to a single load.
Reference is now made to FIG. 2 which shows a three phase AC input supplied to a circuit. This circuit includes three input rectifier filters 10A, 10B and 10C, each connected to receive one phase of the three phase AC input source. Each of these rectifier filters may take the form as the input rectifier filter 10 illustrated in FIG. 1. Each rectifier filter supplies a rectified signal to one of the pre-regulators 12A, 12B and 12C. Each of these regulators may take the form of the PFC pre-regulator 12 illustrated in FIG. 1.
The output signals from the pre-regulators are supplied to DC-to-DC converters 14A, 14B and 14C. Each of these converters may take the form of the converter 14 in FIG. 1. These converters feed a common current sharing circuit 15 and are ORed together to provide a single output sourcing current to a load 16. It is to be noted that the current sharing circuit supplies current sharing feedback to the converters in the manner illustrated in FIG. 2. This is a control loop that operates at a slow speed to ensure that the current from each power supply system is shared equally. Each DC-to-DC converter is used to provide one-third of the total output current. The feedback loop is prone to oscillation and the current sharing between each power supply is, therefore, unbalanced. This drive is required for a larger output power capability and additional system cost.